Crystallisation and fast cooling of the (meta)gabbro from the Chenaillet ophiolite (Western Alps): In-situ U[sbnd]Pb dating of zircon, titanite, monazite and xenotime in textural context

Abstract

The Chenaillet ophiolite, South of Montgenèvre (France), represents a preserved portion of the alpine ocean. The gabbros form lenticular bodies 50 to 200 m thick and a few hundreds of meters wide. They are intrusive in the serpentinites and often overlain by pillow-lavas. Plagiogranite/albitite veins are rare and volumetrically negligible. The petrology of the gabbro s.s. show great details of the progressive cooling of the massif, clinopyroxene are often rimmed by brown to green amphibole +/− ilmenite/titanite. Thermometry on these amphibole assemblages indicates a retrograde temperature evolution from late magmatic to subsolidus, between 950 and 900 °C, 800–750 °C, 600–500 °C. In a shear zone, a string of titanite associated with monazite and xenotime +/− ilmenite is located in a millimetric greenschist layer whose temperature is estimated at 600–550 °C. Accessories chemistry in this sample allowed in situ dating by LA-ICPMS on thin section in textural context. In the same sample, we performed UPb zircon dating, constraining the magmatic age, as well as UPb monazite, xenotime and titanite dating for the metamorphic evolution. In a Tera-Wasserburg diagram, monazite, xenotime and titanite yield 161.3 +/− 4.0 Ma, 161.5 +/− 2.4 Ma and 158.4 +/− 2.3 Ma lower intercept ages, respectively, while the zircon from the same sample indicates an age of 161.0 +/− 0.8 Ma. Finally, zircon dated in an albitite vein also yields a 161.8 ± 1.7 Ma age. These similar ages within the analytical uncertainties show that magmatism and metamorphism were concomitant. Thermal modelling predicts that a 150 m thick gabbroic massif crystallizes in a few hundred years and cools down in several thousand years. This is consistent with the ages reported in our study showing that the retrograde metamorphism in this massif records fast cooling. The development of this retrograde metamorphism required multiple fluid infiltrations during cooling. In the studied samples, 4 types of titanites could be distinguished based on their chemistries. 3 types variously depleted in REE are interpreted as related to water-driven fluids while another type has up to 10% REE, Y and Nb and suggest a possible felsic source. The two potential sources of water promoting this fast cooling are sea water-derived fluids and magmatic fluids exsolved from felsic veins.